Silver iodide has large light absorption in the blue exposure region as compared with silver chloride or silver bromide and this is a promising material for the elevation of sensitivity. However, because of difficulties in the development or chemical sensitization, silver iodide is scarcely used by itself as a photosensitive material, though a mixed crystal thereof is used. As for the technique of positively using silver iodide, U.S. patents by Joe E. Maskasky are known. These patents are mainly characterized by using silver iodide for epitaxially growing silver bromide or silver chloride on the silver iodide grain but the silver iodide grain itself has no particular feature as means for elevating the sensitivity (see, for example, U.S. Pat. No. 4,142,900). The characteristic features of silver iodide grain, which are desired for the elevation of sensitivity, are not yet clearly known.
In the thermal image-forming system using silver halide as a photocatalyst, the size of silver halide grain is preferably made small so as to obtain a high image quality with excellent sharpness and graininess. However, if the grain size is made small, this is accompanied by problems, that is, reduction in sensitivity, enlargement of size distribution and dissolution of grain.